When an acoustic signal is picked up in an environment having reverberation, the original signal is observed with reverberation superimposed thereon. The superimposed reverberation component strongly degrades the clearness of the acoustic signal, making it difficult to extract the original characteristics of the acoustic signal. If an acoustic signal containing reverberation is recognized by an automatic speech recognition (hereinafter called speech recognition) system, for example, the influence of the reverberation significantly lowers the recognition rate of the speech recognition system.
Dereverberation processing is a technique that can eliminate the superimposed reverberation and can restore the original quality of the acoustic signal in such an environment. This makes it possible to restore the clearness of the acoustic signal and improve the speech recognition rate or the like.
Related art of dereverberation processing for eliminating long reverberation includes a method disclosed in Non-patent literature 1.
In the related art, late reverberation is assumed to be attenuated exponentially; an exponential function is used to estimate the energy of late reverberation; and reverberation is eliminated accordingly. More specifically, the observed signal is converted to a frequency-domain signal; the frequency-domain signal is assumed to be the sum of direct sound and late reverberation at each frequency; a model representing reverberation whose energy decreases exponentially (multi-band decay model) is used to estimate model parameters; and late reverberation is eliminated by applying spectral subtraction to both the estimated late reverberation energy and the observed signal energy. An end portion of the sound where just the late reverberation is observed is used to estimate the model parameters.
Non-patent literature 1: I. Tashev and D. Allred, “Reverberation Reduction for Improved Speech Recognition,” 2005 Joint Workshop on Hands-Free Speech Communication and Microphone Arrays.